Update package files & metadata

[whymarrh]

Refs #79

This PR updates the package metadata, post TypeScript:

• Add .editorconfig file
• Add .gitattributes file
• Update npm package structure (via .files in the package.json, diff below)

npm package structure

Before:

$ npm pack
npm notice 
npm notice 📦  eth-sig-util@2.5.2
npm notice === Tarball Contents === 
npm notice 790B   package.json          
npm notice 37B    circle.yml            
npm notice 17.1kB index.js              
npm notice 2.9kB  README.md             
npm notice 398B   .circleci/config.yml  
npm notice 35.5kB test/index.js         
npm notice 5.3kB  utils/eccrypto-lite.js
npm notice === Tarball Details === 
npm notice name:          eth-sig-util                            
npm notice version:       2.5.2                                   
npm notice filename:      eth-sig-util-2.5.2.tgz                  
npm notice package size:  13.4 kB                                 
npm notice unpacked size: 62.1 kB                                 
npm notice shasum:        42b5c3ef87f55db76e73dd3f7c482f0d42eb17d8
npm notice integrity:     sha512-9WPFX01ILINqW[...]dXZedLamAzMZQ==
npm notice total files:   7                                       
npm notice 

Which is confirmed on a fresh download from npm:

$ la node_modules/eth-sig-util/
total 64 
-rw-r--r-- 1 README.md
-rw-r--r-- 1 circle.yml
-rw-r--r-- 1 index.js
-rw-r--r-- 1 package.json
drwxr-xr-x 3 test
drwxr-xr-x 3 utils

After:

$ npm pack
npm notice 
npm notice 📦  eth-sig-util@2.5.2
npm notice === Tarball Contents === 
npm notice 1.0kB  package.json          
npm notice 7.2kB  index.d.ts            
npm notice 20.4kB index.js              
npm notice 2.8kB  README.md             
npm notice 5.3kB  utils/eccrypto-lite.js
npm notice === Tarball Details === 
npm notice name:          eth-sig-util                            
npm notice version:       2.5.2                                   
npm notice filename:      eth-sig-util-2.5.2.tgz                  
npm notice package size:  8.3 kB                                  
npm notice unpacked size: 36.8 kB                                 
npm notice shasum:        da03834127ce49a1f6426588026267eb477a2084
npm notice integrity:     sha512-xaCswPeX2c5Ln[...]Dps0xev46J6KQ==
npm notice total files:   5                                       
npm notice 

[Gudahtt]

LGTM!